Flash Memory Back Pattern Effect Compensation Circuit
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Solution Overview
Problem
In flash memory devices, the 'back pattern effect' causes variations in series string resistance due to programmed states of memory cells above the target cell, leading to unpredictable threshold voltage changes, which complicates accurate data reading and writing.
Innovation Solution
A circuit and method using N latches on each bit line to measure and compensate for the back pattern effect by adjusting the bit line discharge time during read operations, ensuring accurate threshold voltage determination by accounting for varying resistance in the series string of memory cells.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the resistance of series string is varied due to programmed pattern of memory cells, then the bit line discharge rate changes, but this causes the appearance of higher threshold voltage for read cell which reduces measurement precision
Solution Approach 1:
The patent applies preliminary action by performing a read operation on all word lines above the target word line before reading the target cell. This preliminary read captures the back pattern effect (resistance variations) caused by programmed cells in advance. The measured discharge times from these preliminary reads are stored and later used to compensate the threshold voltage measurement of the target cell, thereby eliminating the measurement error before it affects the final reading accuracy.
2Measurement precision
If N latches are added to each bit line to measure back pattern effect, then measurement precision improves, but device complexity increases
Solution Approach 1:
The patent applies universality by designing the N latches to serve multiple functions: they store the back pattern measurement data from preliminary reads, hold the target cell read data, and provide compensation information for threshold voltage correction. This multi-functional design allows precise measurement of back pattern effects without requiring separate dedicated circuits for each function, thereby improving measurement precision while minimizing the increase in device complexity.
3Measurement precision
If bit line discharge time is adjusted to compensate for back pattern effect, then threshold voltage determination accuracy improves, but operation time increases
Solution Approach 1:
The patent applies periodic action by structuring the read operation into distinct phases: a preliminary read phase where all word lines above the target are read to capture back pattern effects, followed by a target read phase where the actual target cell is read with compensation applied. This periodic structure allows the system to efficiently gather compensation data and apply it to the target reading, improving threshold voltage determination accuracy while managing the total operation time through systematic phasing of measurements.
Data Source
AI summary
In one or more of the disclosed embodiments, a read operation is compensated for back pattern effect. A bit line current is generated by a read operation that biases the word lines. As part of a back pattern effect measurement phase, at predetermined time intervals an indication of the discharge status of the bit line is stored in a latch of a set of N latches coupled to each bit line. At the end of the measurement phase, the set of latches contains a multiple bit word that is an indication of the back pattern effect experienced by that particular series string of memory cells. This back pattern effect indication is used in subsequent read operations to adjust the timing of the operation.